The present invention relates to pilot operated hydraulic valves, and more particularly to hydraulic control systems which incorporate such valves along with safety valves.
Construction machinery have moveable members which are operated by a hydraulic cylinder and piston arrangement. The cylinder is divided into two internal chambers by the piston and selective application of hydraulic fluid under pressure to one of the chambers moves the piston in a corresponding direction.
Application of hydraulic fluid to the cylinder typically is controlled by a manual valve, such as the one described in U.S. Pat. No. 5,579,642. In this type of valve, a manual operator lever was mechanically connected to a spool within the valve. Movement of the spool into various positions with respect to cavities in the valve body enables pressurized hydraulic fluid to flow from a pump to one of the cylinder chambers and be drained from the other chamber. By varying the degree to which the spool was moved, the rate of flow into the associated chamber can be varied, thereby moving the piston at proportionally different speeds.
There is a current trend with respect to construction equipment away from manually operated hydraulic valves toward electrically controlled devices that use solenoid valves. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located in the operator cab. This change in technology also facilitates computerized control of various machine functions.
Solenoid valves are well known for controlling the flow of hydraulic fluid and employ an electromagnetic coil which moves an armature in one direction to open a valve. Either the armature or a valve member is spring loaded so that the valve closes when the current is removed from the solenoid coil. As a result, solenoids conventionally are not used to operate a standard valve spool which requires proportional movement in two directions in order to raise and lower a hydraulic cylinder at controlled rates.
Instead, systems have been devised which utilize a pair of solenoid operated valves for each cylinder chamber to be power driven. For a given cylinder chamber, one solenoid valve controls the application of fluid under pressure from a pump to move the piston in one direction, and the other solenoid valve is alternatively opened to drain the fluid from the chamber to a reservoir tank to move the piston in the opposite direction. Thus, if both chambers of a cylinder chambers are to be power driven, four such solenoid valves are required, two supply valves and two drain valves.
The degree to which the valve opens determines the hydraulic pressure at the output port and ideally is proportional to the electric current applied to the solenoid. However, a number of factors, such as load force, valve age and supply pressure, affect the actual pressure produced at any given open position of the valve. Therefore, it is desirable to be able to sense the pressure and produce an electrical signal representative of that sensed pressure. That signal can be used as a feedback signal to the circuit that controls the current to the solenoid. The obvious approach to sensing the pressure would be to provide a pair of ports, one for each output port, in the valve housing at which to attach a pair of pressure sensors. However, that requires additional machining and increased leak potential.